Sean Morrison's laboratory studies the mechanisms that regulate stem cell self-renewal in the hematopoietic and nervous systems and the role these mechanisms play in cancer. Self-renewal is the process by which stem cells divide to make more stem cells, perpetuating stem cells throughout life in adult tissues. Cancers arise from the inappropriate or ectopic activation of these self-renewal mechanisms. Current work in the lab includes the identification of new growth factors that regulate stem cell function and the characterization of aspects of cellular physiology that have not previously been studied in stem cells.
We study the mechanisms that regulate stem cell self-renewal in the hematopoietic and nervous systems and the role these mechanisms play in cancer. Self-renewal is the process by which stem cells divide to make more stem cells, perpetuating stem cells throughout life to regenerate adult tissues. We discovered a series of key regulators of stem cell self-renewal that distinguish it from the proliferation of restricted progenitors in the same tissues. We also identified ways in which self-renewal mechanisms change with age, conferring temporal changes in stem cell properties that match the changing growth and regeneration demands of tissues. Our studies suggest that cancers arise from the inappropriate or ectopic activation of these self-renewal mechanisms, and therefore, that the mechanisms that are competent to cause cancer also change with age. Thus, we study the self-renewal of normal stem cells and the self-replication of developmentally related cancers to understand the physiological function of self-renewal and the ways in which ectopic activation can promote tumorigenesis.
Morrison Research Abstract Slideshow
Figure 1: Wild-type neural crest stem cells (NCSCs) engraft and form neurons as efficiently in the aganglionic region of endothelin receptor B (Ednrb)-deficient gut as in wild-type gut, demonstrating that the Ednrb-deficient hindgut is permissive for the survival and differentiation of neural progenitors. This suggests a possible new strategy for treating Hirschsprung disease, a birth defect that is sometimes caused by mutations in Ednrb and that is associated with a failure to form enteric ganglia in the hindgut. Ednrb is required to modulate the response of NCSCs to migratory cues; loss of Ednrb leads to a failure of these cells to migrate into the hindgut, despite the fact that normal numbers of NCSCs are maintained throughout development in the proximal gut. The migration defect can be bypassed by transplanting NCSCs into the aganglionic region of the Ednrb-deficient embryonic gut.
A: transplanted alkaline phosphatase–expressing neural crest cells (purple) that have engrafted in the hindgut wall of an Ednrb-deficient rat. B: transplanted alkaline phosphatase–expressing neural crest cells (green) that have engrafted and formed neurons (red) in the hindgut wall of an Ednrb-deficient rat. This work demonstrates the ability to gain new insights into the etiology of disease as well as the ability to identify new potential therapeutic approaches by studying the regulation of stem cell function.
From the Morrison lab. See also Iwashita, T. et al. 2003 Science 301:972–976; and Kruger, G.M. et al. 2003 Neuron 40:917–929.
Figure 2: Bmi-1-deficient neural stem cells exhibit a reduced rate of proliferation. Cells within Bmi1–/– stem cell colonies divide at a reduced rate, leading to reduced colony size, a reduced rate of BrdU incorporation into cells, and reduced stem cell self-renewal. BrdU (black stain) is a thymidine analog that is incorporated into DNA when cells divide and therefore marks cells that have recently divided. Bmi-1 is necessary for adult stem cells from the hematopoietic and nervous systems to self-renew normally. In the absence of Bmi-1, these stem cells are formed in normal numbers, but begin expressing senescence pathways postnatally and do not persist into adulthood.
From Molofsky, A.V. et al. 2003 Nature 425:962–967. © 2003 Nature Publishing Group.
Figure 3: The identification of neural crest stem cells (NCSCs ) in the adult gut (enteric nervous system), with p75 (neurotrophin receptor) staining (blue) in a transverse section of the postnatal rat gut. NCSCs were isolated from the adult gut by flow-cytometric purification of the cells that expressed the highest levels of p75. In this section, the cells that stained brightly for p75 localized to the myenteric and submucosal plexi of the enteric nervous system. Prior to this work it was thought that NCSCs terminally differentiated during fetal development and did not persist in the adult peripheral nervous system.
Cover image, Neuron, August 15, 2002. © 2002, with permission from Elsevier. See also Kruger, G.M. et al. 2002 Neuron 35:657–669.
Figure 4: The soluble Notch ligand, Delta-Fc, causes glial lineage determination by neural crest stem cells within 1 day in culture. In this experiment we compared the ability of Delta-Fc to cause glial lineage determination relative to Neuregulin-1, another factor that promotes glial lineage determination. Cultures of purified neural crest stem cells (NCSCs) at clonal density were supplemented either with Delta-Fc (A–C) or with Neuregulin-1 plus the control Fc protein (D–F). After only 24 hours, the cultures were washed into standard medium supplemented with bone morphogenetic protein-2 (BMP2; 50 ng/ml) and grown for 4 more days to test neuronal potential. BMP2 instructs NCSCs to undergo neuronal differentiation. Thus this experiment tests whether transient exposure to either Delta-Fc or Neuregulin-1 causes glial lineage determination quickly enough to prevent NCSCs from undergoing neuronal differentiation in response to BMP2 during the subsequent 4-day culture period.
Panels A and D show phase-contrast images, panels B and E show bright-field images of staining with the neuronal marker peripherin, and panels C and F show superimposed epifluorescence images of staining with the glial marker glial fibrillary acidic protein (GFAP, red) and the myofibroblast marker smooth muscle actin (SMA, green, negative). The control colony preincubated in Neuregulin-1 plus Fc (D–F) contained mostly neurons and neuronal precursors induced by the BMP2 treatment, as judged by the peripherin staining and the lack of GFAP and SMA staining. In contrast, the colony preincubated in Delta-Fc (A–C) contained only glia (red) despite the BMP2 treatment, as indicated by the GFAP staining and the lack of peripherin and SMA staining. Thus Delta-Fc acted more rapidly than Neuregulin-1 to instruct gliogenesis.
From Morrison, S.J. et al. 2000 Cell 101:499–510. © 1999 from Elsevier Science.
Figure 5: Neural crest stem cells (NCSCs) were isolated by flow cytometry from the sciatic nerves of embryonic day 14 rat fetuses as cells that expressed the neurotrophin receptor p75 but failed to express the myelin protein P0. These cells were cultured at clonal density for 14 days, then stained with antibodies against neuronal (peripherin, black), glial (GFAP, red), and myofibroblast (smooth muscle actin, green) markers. Single purified NCSCs gave rise to multilineage colonies like this one, containing more than 100,000 cells after only 14 days of culture.
From Morrison, S.J. et al. 1999 Cell 96:737–749. © 1999 from Elsevier Science.
In addition to studying the cell-intrinsic mechanisms that regulate self-renewal, we also study the extrinsic mechanisms by which the niche, or microenvironment, regulates hematopoietic stem cell (HSC) maintenance. We discovered that bone marrow HSCs are maintained in a perivascular niche in which endothelial cells and leptin receptor-expressing stromal cells secrete the factors that promote stem cell maintenance. The identification of cell-intrinsic and cell-extrinsic mechanisms that regulate self-renewal have set the stage for us to identify new growth factors that regulate stem cell function in the niche, as well as the regeneration of hematopoietic cells and bone throughout life.
Stem Cell Self-Renewal
The maintenance of many adult tissues depends upon the persistence of stem cells throughout life. Stem cells are maintained in adult tissues by self-renewal, the process by which stem cells divide to make more stem cells. By better understanding this process we gain insights into how tissues develop and regenerate, how reduced self-renewal can lead to degenerative disease, and how increased self-renewal can lead to tumorigenesis. We have discovered that networks of proto-oncogenes and tumor suppressors that control cancer cell proliferation also regulate stem cell self-renewal, but that these networks do not generically regulate the proliferation of all cells. Restricted progenitor proliferation does not require many of the mechanisms that regulate stem cell self-renewal.
We have historically taken forward and reverse genetic approaches to identify new genes that are required for stem cell self-renewal. Each time we identify a self-renewal regulator we learn something new about how self-renewal occurs by examining the downstream mechanisms. For example, we have identified a network of heterochronic gene products that regulates stem cell maintenance throughout life, while also regulating the temporal changes in stem cell properties that are required to match the changing growth and regeneration demands of fetal and adult tissues.
To go beyond traditional studies of individual gene products we are now developing new methods to study aspects of cellular physiology, such as the regulation of proteostasis and metabolism, that have been studied only to a limited extent in somatic stem cells. Studies of these mechanisms in stem cells have the potential to reveal ways in which they are used differently by different kinds of dividing somatic cells and how these differences may be necessary for the maintenance of tissue homeostasis. These studies may also provide general insights into the extent to which these mechanisms differ in extensively self-replicating cells as compared to other cells.
We also study the extrinsic mechanisms by which the niche regulates stem cell maintenance. Our studies of the niche focus on the hematopoietic system, where we have discovered that quiescent hematopoietic stem cells (HSCs) reside in a perivascular niche in which endothelial cells and leptin receptor–expressing perivascular stromal cells secrete the known factors that promote HSC maintenance. The discovery and characterization of this niche has allowed us to identify new mechanisms by which HSCs and the niche regulate each other, including the identification of new growth factors and the ways in which the niche changes in response to injury.
Stem Cell Aging
Much of age-related morbidity in mammals may be determined by the influence of aging on stem cell function. We have found that stem cells from the hematopoietic and nervous systems undergo strikingly conserved changes in their properties as they age, including declining self-renewal capacity.
We discovered that the networks of heterochronic gene products that regulate temporal changes in stem cell properties between fetal and adult stages (see above) also regulate stem cell aging. For example, Hmga2 expression declines while let-7 expression and Ink4a expression increase with age, reducing stem cell frequency and function in multiple tissues. By deleting Ink4a from mice, we partially rescued the decline in stem cell function with age and enhanced the regenerative capacity of aging tissues. Networks of proto-oncogenes and tumor suppressors thus change throughout life to balance tissue regeneration with tumor suppression: proto-oncogenic signals dominate during fetal development when tissue growth is rapid but cancer risk is low, and tumor-suppressor mechanisms are amplified during aging when there is little tissue growth but cancer risk is high. By developing the ability to study aspects of cellular physiology that have not been studied before in stem cells, we expect to gain insights into the aging of regenerative tissues. For example, do proteostasis mechanisms differ between extensively self-renewing stem cells, versus cells with limited replicative potential and postmitotic cells, and to what extent is tissue regeneration during aging limited by failures of proteostasis in stem cells?
The Self-Replication of Cancer Cells
Cancer cells hijack stem cell self-renewal mechanisms by acquiring mutations that overactivate these pathways. What does this mean for aspects of cellular physiology that differ between stem cells and other cells? Does the extensive replicative capacity of cancer cells depend upon the same differences? Or do the cancer cells acquire independence from some of the constraints on normal stem cells as they mutate tumor-suppressor mechanisms? Our studies sometimes reveal new vulnerabilities in cancer cells that could be exploited in anticancer therapies. For example, ion gradients are rarely studied in cancer cells. However, we have discovered that the ability of cancer cells to maintain subcellular ion gradients appears to be persistently stressed and that inhibitors of ion transporters can have synthetic lethal effects when combined with targeted agents that inhibit oncogenic signaling pathways.
This research has been supported in part by grants from the National Institute on Aging; the National Institute of Neurological Disorders and Stroke; the National Heart, Lung, and Blood Institute; the National Institute of Diabetes and Digestive and Kidney Diseases; and the Cancer Prevention and Research Institute of Texas.
As of April 19, 2016